System safety assessments combining first principles and model based safety assessment methodologies

In performing assessments of low probability, high consequence systems, it is often preferable to use more than one methodology in order to assure that such systems undergo a thorough assessment. Hence, employing two methodologies in a complementary manner allows the analyst to bring the strongest features of each approach to bear upon the problem. The results of one methodology can be used to crosscheck or better characterize the results of another methodology, with the results being synergized in providing a comprehensive assessment of the system. This paper will briefly describe both the first principles and model based safety assessment methodologies, and will illustrate how both methods are used in a complementary manner in order to perform overall safety assessments of low probability, high consequence engineered systems at Sandia National Laboratories.